The present application relates to and incorporates herein by reference Japanese Patent Application No. 11-271576 filed on Sep. 27, 1999.
The present invention relates to an air-fuel ratio control system and method for controlling fuel injection amount using a control model of an internal combustion engine that simulates a control object between a fuel injection point and an air-fuel ratio detection point of the engine.
Internal combustion engines of vehicles are controlled in a closed-loop or feedback manner with respect to air-fuel mixture supply. Specifically, the engine has a three-way catalyst at its exhaust side, and an air-fuel ratio sensor is provided upstream the catalyst. The air-fuel ratio of mixture, that is, the amount of fuel, supplied to the engine at the engine intake side is controlled to a target air-fuel ratio such as the stoichiometric ratio in response to air-fuel ratio detection outputs of the sensor.
In U.S. Pat. No. 5,445,136, it is proposed to simulate as a control object the engine covering from the fuel injection point to the air-fuel ratio detection point, and determines a calculation equation for calculating an air-fuel ratio correction coefficient from the simulated model. The air-fuel ratio correction coefficient is repetitively updated by substituting into the equation a deviation of the detected air-fuel ratio from the target air-fuel ratio and air-fuel ratio correction coefficients used previously. The fuel injection amount is calculated by correcting basic fuel injection amount with the updated air-fuel ratio correction amount.
The response time constant or delay of the control model of the engine from the fuel injection point to the air-fuel ratio detection point varies with engine operation conditions, particularly the intake air amount of the engine. This is because the response characteristics of the air-fuel ratio sensor that greatly affects the response characteristics of the control model varies with the engine operation conditions, particularly the intake air amount. For instance, the response time constant of the air-fuel ratio sensor becomes larger and, as a result, the response time constant of the control model becomes larger as the intake air amount decreases.
The conventional control models have not been determined in view of changes in the response time constant resulting from changes in the engine operation conditions. The control gain therefore had to be set relatively small so that the engine may be operated with stableness over entire operation range. The small control gain lessens the response characteristics of the air-fuel ratio control relative to changes in the engine operation conditions, resulting in insufficient exhaust gas purification by the catalyst.
It may be possible to switch the control model from one to another of a plurality of control models each time the engine operation condition changes from one range to another. However, this model switching will tend to generate discontinuities between the control model characteristics, and hence the air-fuel ratio correction coefficients calculated based on the determined control model will largely change at the time of model switching. This large change also results in deviation of the actual air-fuel ratio from the target air-fuel ratio, causing insufficient exhaust gas purification in the catalyst.
It is therefore an object of the present invention to provide an air-fuel ratio control system and method capable of changing a control model of engine without using a plurality of control models. According to the present invention, an air-fuel ratio control system has a fuel injector for injecting fuel into an engine and an air-fuel ratio sensor for detecting an air-fuel ratio of air-fuel mixture supplied to the engine. The engine is simulated mathematically as a control model that covers from a fuel injection point to an air-fuel ratio detection point. A response time constant of the control model is calculated as a continuous function of a predetermined engine operation parameter variable with a flow of air-fuel mixture, and a control gain of the control model is calculated as a continuous function of the calculated response time constant. Control parameters are calculated from the calculated response time constant and the calculated control gain, and an air-fuel ratio correction coefficient is calculated using the calculated control parameters and a deviation of the detected air-fuel ratio from a target air-fuel ratio. A fuel injection amount is calculated based on engine operating conditions and the calculated air-fuel ratio correction coefficient.